It would be fun but yeah it would most definitely be too OP. Armour and tools are annoying to replace but it's just one of those things I guess (and it's probably vital for the enchantment economy and general EMC economy )

Having Soulbound on something would be a great idea if it was properly priced (expensive) and removed the ability to repair it. While the idea of a soulbound item that can forever be repaired sounds nice, it would further take away the 'loss' factor that you NEED to remain a pure survival server.

Also, think about other options (not limiting it to just tools) like adding soulbound to some stacks of diamond blocks you may be carrying from one place to another in the wild.